Field of the Invention
The invention relates to a temperature compensation circuit for a Hall element for magnetic field measurement.
Hall sensors for magnetic field measurement must have a high precision. This is counteracted by the fact that temperature influences result in temperature dependencies, and discrepancies in manufacturing parameters result in manufacturing-dependent and technology-dependent discrepancies, which mean that Hall sensors having a digital output signal have discrepancies from the nominal switching points.
Various approaches are known for trimming out and for compensating for temperature-dependent or manufacturing-dependent tolerances.
An integrated Hall circuit with an adjustable operating point is specified in the U.S. Pat. No. 4,705,964. Adjustable resistors are provided for this purpose, and are formed by series circuits of resistance elements, with fusible links that are used to adjust the resistances. On one hand, complex measurement methods are required for this purpose, while on the other hand, the described principle occupies a large area on the chip surface and involves a high level of complexity for trimming.
Comparable trimming methods such as Zener zapping, laser fuses and laser trimming methods have similar disadvantages.
A temperature-compensated Hall effect sensor is specified in U.S. Pat. No. 4,833,406. In this case, a bipolar difference input stage is arranged at the output of the Hall element, for temperature compensation. However, there is a high degree of non-linearity in this case, which restricts the accuracy of the trimming process.
A compensated Hall sensor is specified in U.S. Pat. No. 5,260,614. In this case, at least two current sources form a compensating excitation current for supplying the Hall sensor. Complex circuitry is required to produce the excitation current for the Hall sensor.
A temperature compensation method for a Hall effect circuit is specified in Published European Patent Application EP 0 450 910 A2. In this case, a resistor is arranged in the same epitaxial layer as the Hall element, for temperature compensation. Because of the large number of series-connected base-emitter voltages, the circuit requires a high supply voltage, however, which amounts at least to the sum of three base-emitter voltages and a saturation voltage.
The principle of band gap reference is known from Tietze, Schenk: Halbleiter-Schaltungstechnik [Semiconductor circuit technology], 10th Edition, Springer Verlag 1993, page 558.
It is accordingly an object of the invention to provide a temperature compensation circuit for a Hall element which overcomes the above-mentioned disadvantages of the prior art apparatus of this general type.
In particular, it is an object of the invention to provide a temperature compensation circuit for a Hall element, which automatically compensates for manufacturing and temperature fluctuations and which is suitable for operation with low operating voltages.
With the foregoing and other objects in view there is provided, in accordance with the invention, a temperature compensation circuit for a hall element. The temperature compensation circuit has a first band gap reference circuit including a first resistor of a first resistor type, a Hall element for providing a Hall voltage, and a current mirror connecting the Hall element to the first resistor. The first resistor has a first resistance. A first reference voltage is dropped across the first resistance of the first resistor during operation. The current mirror feeds an excitation current into the Hall element. The excitation current is proportional to the quotient of the first reference voltage and the first resistance. The temperature compensation circuit also has a second band gap reference circuit including a second resistor of a second resistor type. A second reference voltage is dropped across the second resistor during operation. The temperature compensation circuit also has a comparator including inputs connected to the Hall sensor and to the second resistor for comparing the Hall voltage and the second reference voltage.
Providing two band gap reference circuits which use different resistor types has the advantage that it allows a magnetic field signal to be detected and digitized, independently of the technology, thus independently of the manufacturing parameters, and in a determined manner with respect to the temperature, without any additional trimming.
A further advantage of the present temperature compensation circuit is that it operates largely independently of the operating voltage of the temperature compensation circuit, and still functions at very low operating voltages.
If the temperature compensation circuit is intended to be used to produce a digital output signal, then the present circuit can be used to produce switching points that are largely independent of manufacturing and temperature parameters.
The first reference voltage, which is dropped across the first resistor, is transformed by the current mirror in the first band gap reference circuit to a supply voltage for the Hall element, and the supply voltage is proportional to the first reference voltage.
The first resistor may in this case be technologically coupled to the Hall element. For example, the first resistor may be arranged in the same epitaxial layer as the Hall element. The Hall element supply voltage that can be produced in this way is in consequence independent of the technology, since doping parameters and layer thickness discrepancies during manufacture are the same both in the first resistor and in the Hall element.
During production of the temperature compensation circuit, there is no need for temperature trimming or for sensitivity trimming.
In one advantageous embodiment of the present invention, the second band gap reference circuit has a third resistor, which is of the second resistor type, and is connected to the comparator in order to supply a third reference voltage that is dropped across the third resistor. There is a temperature coefficient that is approximately independent of the manufacturing technology between the second and the third reference voltages. Any desired voltage with any desired temperature coefficient and temperature profile can be produced independently of the technology by weighted adding or subtraction of the second and third reference voltages. The second band gap reference circuit can be used, for example, to produce switching points for comparators for digitally further-processing the Hall voltage signal.
Both the first and the second reference voltage are band gap reference voltages. These can be produced, for example, from the difference between two base-emitter voltages. These can be produced using two transistors that are operated with different current densities.
The third reference voltage may be a base-emitter voltage.
In a further preferred embodiment of the present invention, four transistors, which together form a feedback loop, are provided for producing the first reference voltage. The transistors are connected to a reference ground potential, to a supply potential and to the first resistor. The transistors are in this case connected such that a PTAT (proportional to absolute temperature) voltage is formed across the first resistor, which is connected to the emitter of the second transistor. The transistors are in this case connected to one another such that only in each case one base-emitter voltage or one threshold voltage of an MOS transistor is dropped between the supply potential and the reference ground potential, so that the circuit can be operated with a low supply voltage.
In a further advantageous embodiment of the present invention, the fourth transistor also forms a current mirror with a fifth transistor, which is connected to the Hall element.
The current mirror is used to mirror the current that is formed from the quotient of the first reference voltage and the first resistance, in order to supply an excitation current to the Hall element. This current mirror may have amplifying characteristics, so that the voltage that is dropped across the Hall element is amplified in comparison to the first reference voltage.
Instead of the fifth transistor, an amplifier circuit or current mirror circuit may be provided, which is designed such that a voltage is dropped across the Hall element and this voltage is proportional to a weighted product of the first reference voltage and a base-emitter voltage of a transistor in the band gap reference circuit. The factors may be weighted with weighting factors, which for example, can be adjusted by using the current mirror ratios.
In a further advantageous embodiment of the present invention, a second current mirror is connected to the second resistor, in order to output a second current, which is proportional to the second reference voltage.
In a further advantageous embodiment of the present invention, the third resistor is connected to the comparator such that the comparator can be supplied with a sum voltage of the second and third reference voltages, for comparison with the Hall voltage. The second and third reference voltages can in this case be weighted with weighting coefficients such that any desired temperature coefficient or complex higher-order temperature profiles can be achieved.
In a further advantageous embodiment of the present invention, the comparator is preceded by at least one operational amplifier to amplify the Hall voltage and the reference voltages.
While the first resistor and the Hall element are of a first resistor type, in which case both can be formed using the same semiconductor technology, and in particular, may have the same vertical structure, the second and third resistors are of a second resistor type, which is not the same as the first resistor type. The second and third resistors may, for example, be formed in a polycrystalline silicon layer, while the first resistor and the Hall element are in the form of an epitaxial or trench resistor.
The second and third resistors may be a diffusion resistor.
The amplifier may have MOSFET or JFET input stages. The gain of the amplifier can be adjusted via the resistance ratios and is therefore not influenced by the gradient of bipolar difference input stages and their non-linearity.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a Temperature compensation circuit for a Hall element, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.